Current hearing-aid technology focuses on amplification, which is a manipulation of the magnitude (amplitude) spectrum of a sound. Typical hearing aids amplify to compensate for loss of gain and/or sensitivity in the cochlea, but they do not purposefully manipulate the phase spectrum. Instead, most hearing aids attempt to restore the quality of sound for hearing-impaired listeners by amplifying the sound in a frequency-dependent scheme that is based on a listener's hearing ability (thresholds) at different frequencies, i.e., if there is more hearing loss at high frequencies, more amplification is applied at high frequencies. Additionally, the amount of amplification is often varied with the sound-level in a compressive manner in order to compress the wide dynamic range of sound into the limited dynamic range of hearing-impaired listeners, e.g., the WDRC (wide dynamic range compression) strategy.
Most amplification strategies are variations and/or combinations of different schemes for controlling gain across frequency, i.e., using different numbers of frequency channels that can be independently controlled, and for varying the compression across the frequency channels. All of these strategies are focused on manipulating the magnitude spectrum of the acoustic stimulus, but they do not include purposeful manipulation of the phase spectrum.
In the past decade, WDRC hearing aids have gained some success in restoring normal loudness perception in hearing-impaired listeners by giving low-level inputs relatively more gain than high-level inputs. However, discrimination and identification of complex sounds, such as speech, cannot be fully restored by the adjustment of gain, i.e., the magnitude spectrum.
In the healthy ear, the phases of phase-locked auditory-nerve (AN) responses change systematically with level. Discharge times across fibers tuned to a range of frequencies near a stimulus frequency become more similar as the input level is increased and less so when the input level is decreased. In the impaired ear, peripheral filters are broader, and therefore response times are more similar across frequencies even at low input levels. The properties of the phase spectrum remain to be incorporated into signal-processing strategies.